The present invention relates to tantalum powder, in particular, tantalum powder used as an anode material for electrolytic capacitors, an anode body for electrolytic capacitors which is produced while making use of the tantalum powder and an electrolytic capacitor which comprises the anode body incorporated therein.
Tantalum powder for use in a tantalum solid capacitor is in general produced through the reduction of potassium tantalum fluoride (K.sub.2 TaF.sub.7) with sodium metal:
K.sub.2 TaF.sub.7 +5Na.fwdarw.Ta+2KF+5NaF
The tantalum spault produced by the reduction is disintegrated, washed with water to remove the salts as by-products, washed with an acid, dried and then heat-treated. Thereafter, a part of the resulting powder is subjected to oxygen-removal through a treatment with, for instance, magnesium metal for the purpose of reducing the oxygen content thereof to thus give a final product.
To the tantalum powder thus produced, there is added an appropriate binder, followed by pelletting, sintering and formation an anodized film of tantalum pentoxide, on the sintered body surface, through an anodization in an electrolyte such as an aqueous solution of phosphoric acid.
Then manganese dioxide as a solid electrolyte layer is formed on the sintered body surface on which the anodized film has been formed, through thermal decomposition of manganese nitrate, a graphite layer and a layer of a silver paste are further formed on the solid electrolyte layer and then the product is encapsulated with a resin to give a solid electrolytic capacitor. The bonding strength between a tantalum wire serving as a lead wire and the sintered body and the strength of the sintered body are important factors for the preparation of the electrolytic capacitor.
More specifically, when mounting a tantalum capacitor on a substrate through soldering, the resin expands and contracts due to thermal stress and thus the resin may peel off if the bonding strength between the tantalum wire and the sintered body is low. This may result in an increase in the leakage current.
Moreover, the strength of the sintered body should be sufficiently high so that the body can withstand thermal stress and the action of gaseous nitric acid as a by-product generated when manganese dioxide is deposited thereon as a solid electrolyte layer through the thermal decomposition of manganese nitrate.
In general, the formation of a strong bonding between the tantalum wire and the sintered body and the formation of a sintered body having high strength can be ensured by increasing the density during pelletting and raising the sintering temperature. However, an increase in the density during pelletting and a raise in the sintering temperature are accompanied by simultaneous decreases in electrical properties, in particular, a decrease in CV (capacity) and make the immersion of the sintered body in manganese nitrate difficult.